Recently, chance of getting and leading information of documents and images in a form of electronic information simpler than those in a form of usual printed material on paper is more increased accompanied with increasing in the rapidness of the processing speed of personal computer, spreading of network infrastructure and capacity glowing and cost lowering of data storage.
As the means for leading such the electronic information, liquid crystal displays and CRT are usually used and light emission type displays such as an organic electroluminescent are mainly applied recently. However, it is necessary to observe such the leading means for long time, particularly, when the electronic information is information of documents. Such the action is difficultly considered as a gentle means for the human. As the problems of such the light emission type displays, it has been known that eyes are fatigued by flicker, portability is low, posture for leading is limited and focusing on the still image is necessary and electric consumption is increased accompanied with prolongation of the leading time.
Although memory type reflective displays utilizing outside light which do not consume electric power for sustaining the image have been known as the means for compensating such the defects, such the displays are difficultly considered to have sufficient properties in the present condition by the following reasons.
In the system using a polarizing plate such as a reflective liquid crystal plate, the reflectivity is low as about 40% so that display of white image is insufficiently and majority of the production methods to be used for producing the constituting parts of the display is not always simple is utilized. A polymer dispersion type liquid crystal requires high voltage and the contrast of the obtained image is not insufficient since difference of between the refractive indexes of organic compounds. A polymer network type liquid crystal has problems that high voltage is required and a complex TFT circuit is necessary for improving the memorizing ability. A displaying element using electrophoresis requires high voltage such as 10 V or more and has anxiety about lowering in the durability caused by coagulation of the electrophoresis particles. An electrochromic displaying element is insufficient in the color quality of black or color such as yellow, magenta, cyan, blue, green and red, and has anxiety about that complex layer formation such as vapor deposition is necessary for sustaining the memorizing ability though such the element can be driven at low voltage such as 3 V or less.
An electro-deposition system, hereinafter referred to as ED system, applying the dissolution-deposition of metal or metal salt has been known as a method for dissolving such the problems of each of the above systems. The ED system has advantages such as that the element can be driven at low voltage such as 3 V or less, the structure of the cell is simple, the contrast of black and white in the image is suitable and the quality of black image is high, and various methods are disclosed; cf. Patent Documents 1 to 3, for example.
As a result of detailed investigation on the technology disclosed in the above Patent Documents by the inventors it is found that the reflectivity on the occasion of displaying white image and the displaying speed are insufficient for satisfying the recent needs of the consumers. For instance, addition of titanium dioxide to the electrolyte liquid can be cited as a method for controlling the reflectivity on the occasion of displaying white color. However, it is found that undesirable unevenness on the displayed image is caused by coagulation of the titanium dioxide particles when the adding amount of the titanium dioxide is increased. Moreover, it has been known as to the method of solidifying the electrolyte by adding a butyral type resin to the electrolyte that the displaying speed is considerably lowered even though the coagulation of the titanium dioxide is improved; cf. Patent Document 4 and Non-Patent Document 1, for example. Moreover, it is found that other problems such as that leaking of liquid and variation in the displaying property during prolonged using term are caused in the displaying element using the liquid electrolysis under various using conditions practically assumed. As to the problems of the liquid leaking and the property variation during the prolonged using term, a method for gelling the electrolysis by adding a gelling agent such as an ethylene oxide type resin or a gelling agent having a molecular weight of not more than 1,000 is investigated; cf. Patent Documents 5 to 7, for example.
As a result of detailed investigation on the methods disclosed in Patent Documents 5 to 7, it is found that the unevenness on the displayed image caused by coagulation of titanium dioxide is formed during repeating use since the titanium dioxide sustaining ability of the ethylene oxide type resin or the gelling agent having a molecular weight of not more than 1,000 is in sufficient and the displaying speed is considerably lowered when the ethylene oxide type resin or the gelling agent having a molecular weight of not more than 1,000 is added into the electrolyte.    Patent Document 1: U.S. Pat. No. 4,240,716    Patent Document 2: Japanese Registration Patent No. 3428603    Patent Document 3: Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 2003-241227    Patent Document 4: Japanese Registration Patent No. 3623050    Patent Document 5: JP-A No. S57-143356    Patent Document 6: JP-A No. 2004-309946    Patent Document 7: JP-A No. H11-185836    Non-Patent Document 1: N. Kobayashi, N. Nishimura and H. Ohotomo, Polymer Electrolyte with Large Temperature-Dependent Conductivity for Novel Electrochromic Imaging, Electrochim. Acta, 50, 3886-3890 (2005)